The impossibility of treason in the minds of the privileged

'Donald Maclean is a very nice individual indeed and has plenty of brains and keenness. He is nice looking and ought, we think, to be a success in Paris from the social as well as the work point of view.''

This Foreign Office memo recommending Maclean for a posting at the British Embassy in France says much about how, in 1934, four college friends – Anthony Blunt, Guy Burgess, Kim Philby and Maclean – managed to become the most lethally effective double agents of the 20th century. After all, no one suspected that these "nice" public school chaps would do anything treacherous towards their own green and pleasant land.

It's this extraordinary dichotomy – between the politeness of upper-class life and the atrocities that were committed in their name – that Cambridge Spies, BBC2's new four-part serial about our most famous traitors, promises to explore. The main parts are taken by Toby Stephens (Philby), Sam West (Blunt), Tom Hollander (Burgess) and Rupert Penry-Jones (Maclean), who share something of their characters' backgrounds (Hollander and West are Oxbridge boys while Stephens went to Philby's Surrey prep school).

According to producer Mark Shivas, the drama traces the inexorable erosion of the quartet's youthful idealism; in one pivotal moment of the film, General Franco is seen giving the duplicitous Philby a medal for his "heroic actions in the struggle against Communism''. In reality, the British spy has been sent by his Soviet handlers to assassinate the Spanish dictator and is agonising over his next move. On the Barcelona set where this scene is being filmed, there's a palpable sense of tension among the Spanish crew-members.This is one of the first times that Franco has been portrayed in a film shot in Spain, and the locals are not altogether comfortable with the idea. "For the older generation, the Civil War still seems very close," says location manager, Francisco Rojo Villanda. "Yesterday I met an 89-year-old woman who showed me cracks in her front room that were caused by Franco's bombs. She's too poor to have got them repaired. People like her say it's better not to talk about that period at all. Younger people, on the other hand, think its good to show what happened under Franco, and I agree with them. I believe it's important to confront your past by conveying it in a drama ... I think we'll learn a lot from this.''

Written by Peter (North Square) Moffat, Cambridge Spies has not only been provoking debate in Spain; Moffat's account of the sexual proclivities of the four spies has already sparked comment and Oleg Gordievsky, the former KGB colonel who acted as a double agent for MI6, has said that "the films present so distorted a version of the history they claim to portray that they do not tell the story of the Cambridge spies. What they portray is more akin to a piece of KGB propaganda.''

The BBC has its defence well marshalled. "We're not deliberately trying to be controversial,'' declares executive producer Gareth Neame. "But in revisiting this story, we know that some people will say 'these four were downright enemies of the state – how could the BBC make a drama glorifying them?' But we have no axe to grind – we never say they weren't traitors. We just want to examine why they did what they did.''

According to Shivas, Cambridge Spies doesn't try to gloss over their legacy: "At the beginning of this drama, they're shown in a sympathetic light, but later we don't mince words about the consequences of their actions. For instance, Maclean told the Soviets about the atom bomb long before they would have known about it otherwise. Hundreds of people died as a result of the four's actions.''

Dramatists have long been drawn to this treacherous quartet: Alan Bennett focused on them in his screenplays, An Englishman Abroad and A Question of Attribution, Ian Curteis wrote a play entitled Philby, Burgess and Maclean, while Julian Mitchell covered their school days in Another Country. But just what is it that continues to make them such riveting figures? "We're all gripped by that Boy's Own idea of performing dangerous undercover operations," says director Tim Fywell. "Everybody loves James Bond. But this lot are particularly fascinating because of their inherent contradictions. Blunt, for example, was distantly related to the Royal Family. He was the Surveyor of the Queen's pictures and a Soviet spy at the same time. That mass of contradictions is absolutely compelling.''

The sheer implausibility of these four quintessential English gentlemen being Soviet spies is what enabled them to get away with it for so long. "Class is very important in this story,'' Neame observes. "These four had a life of privilege mapped out – they glided effortlessly from quadrangle to quadrangle ... at Eton, Cambridge and the Foreign Office. Because they wore the right tie and knew the right people, they were never rumbled. Nobody every suspected that 'one of us' could possibly be a traitor.''

The first part of 'Cambridge Spies' starts 9 May at 9pm on BBC2